BGP always-compared-med实验
2016-01-14 22:30
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1.实验目的
(1)默认情况下,从不同AS得到同一个NLRI的信息,会忽略MED值。必须开启BGPalways-compared-med命令,才会进行比较。
(2)在至同一个NLRI有多个路径的情况下,BGP路由的优选顺序为local-pref、AS-PATH长度、origin、MED值、neighbortype、IGP metric。在上述依然等价的情况下,再比较RID、neighbor ID。
2.实验结果
(1)默认情况下,未开启BGPalways-compared-med。在如图所示的拓扑中,R4在BGP进程中以network的方式发布环回地址4.4.4.4。
在不手动配置BGPRouter ID的情况下,BGP进程采取与OSPF相同的方式,在双up的环回接口中,选择地址最大的作为自己的RID。但修改RID,只在进程重启的情况下生效,即具有不可抢占性。如表1所示,根据实验目的(2)中说明的优选顺序,R1优选从R2处获取的4.4.4.4 BGP路由,比较依据为R2拥有较低的RID。为R2配置换回接口2,配置地址为22.2.2.2,使用clear ip bgp *命令,使之生效。再在R1上观察4.4.4.4 BGP路由情况,优选R3,因为相比有较低的RID。
但必须注意,在EBGP路由情况下,只是在同时获取的情况下进行比较,否则原有路由优先。如R1优选从R3获取4.4.4.4 BGP路由,此时即便R2加入,有较低的RID,仍然不会发生变化。如表2所示,先关闭R1和R2的BGP邻居,使R1优选从R3获取4.4.4.4 BGP路由。再激活R1和R2的BGP邻居,此时优选顺序并不发生改变。
表1 BGP路由情况(1)
R1#sh ip bgp 4.4.4.4 BGP routing table entry for 4.4.4.4/32, version 2 Paths: (2 available, best #2, table default) Advertised to update-groups: 4 Refresh Epoch 2 3 4 10.0.0.14 from 10.0.0.14 (3.3.3.3) Origin IGP, localpref 100, valid, external rx pathid: 0, tx pathid: 0 Refresh Epoch 2 2 4 10.0.0.2 from 10.0.0.2 (2.2.2.2) Origin IGP, localpref 100, valid, external, best rx pathid: 0, tx pathid: 0x0 !!!!为R2配置换回接口2,配置地址为22.2.2.2,使用clear ip bgp *命令,使之生效。 !!!再在R1上观察4.4.4.4 BGP路由情况,优选R3,因为相比有较低的RID。 R1#sh ip bgp 4.4.4.4 BGP routing table entry for 4.4.4.4/32, version 3 Paths: (2 available, best #2, table default) Advertised to update-groups: 4 Refresh Epoch 1 2 4 10.0.0.2 from 10.0.0.2 (22.2.2.2) Origin IGP, localpref 100, valid, external rx pathid: 0, tx pathid: 0 Refresh Epoch 2 3 4 10.0.0.14 from 10.0.0.14 (3.3.3.3) Origin IGP, localpref 100, valid, external, best rx pathid: 0, tx pathid: 0x0 |
表2 BGP路由情况(2)
R1(config-router)#do sh ip bgp BGP table version is 3, local router ID is 1.1.1.1 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter, x best-external, a additional-path, c RIB-compressed, Origin codes: i - IGP, e - EGP, ? - incomplete RPKI validation codes: V valid, I invalid, N Not found Network Next Hop Metric LocPrf Weight Path *> 4.4.4.4/32 10.0.0.14 0 3 4 i !!!! 激活R1和R2的BGP邻居,此时优选顺序并不发生改变。 R1(config-router)#do sh ip bgp BGP table version is 3, local router ID is 1.1.1.1 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter, x best-external, a additional-path, c RIB-compressed, Origin codes: i - IGP, e - EGP, ? - incomplete RPKI validation codes: V valid, I invalid, N Not found Network Next Hop Metric LocPrf Weight Path * 4.4.4.4/32 10.0.0.2 0 2 4 i *> 10.0.0.14 0 3 4 i |
未开启BGPalways-compared-med命令时,即便经R3获取的路由具有较低的MED值,仍不改变优选情况,与表1一致。
开启BGPalways-compared-med命令时,在R1上观察4.4.4.4 BGP路由,优选MED值较小的R3。
表3 BGP路由情况(3)
R1(config-router)#do sh ip bgp BGP table version is 2, local router ID is 1.1.1.1 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter, x best-external, a additional-path, c RIB-compressed, Origin codes: i - IGP, e - EGP, ? - incomplete RPKI validation codes: V valid, I invalid, N Not found Network Next Hop Metric LocPrf Weight Path * 4.4.4.4/32 10.0.0.14 50 0 3 4 i *> 10.0.0.2 100 0 2 4 i !!!!开启BGP always-compared-med命令,在R1上观察4.4.4.4 BGP路由,优选顺序发生变化,优选MED值较小的R3。 R1(config-router)#do sh ip bgp BGP table version is 2, local router ID is 1.1.1.1 Status codes: s suppressed, d damped, h history, * valid, > best, i - internal, r RIB-failure, S Stale, m multipath, b backup-path, f RT-Filter, x best-external, a additional-path, c RIB-compressed, Origin codes: i - IGP, e - EGP, ? - incomplete RPKI validation codes: V valid, I invalid, N Not found Network Next Hop Metric LocPrf Weight Path *> 4.4.4.4/32 10.0.0.14 50 0 3 4 i * 10.0.0.2 100 0 2 4 i |
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